Typically, motor vehicles are equipped with a suspension system to improve road-adherence and to provide ride-comfort for occupants. The suspension system includes springs and shock absorbers. The shock absorbers are disposed in parallel with the springs to damp the vibration of the springs.
The shock absorbers utilize a fluid flow system incorporating therein either a hydraulic fluid having a constant viscosity or a fluid having a changeable viscosity, e.g., magnetorheological (MR) fluid. The use of MR fluid is advantageous in that the viscosity thereof can be controlled with the application of a magnetic field in order to adjust a damping force being exerted on the springs depending on a traveling condition.
Particularly, MR fluid is a free-flowing liquid with a viscosity. Exposure to a magnetic field can transform the liquid into a near-solid in milliseconds; and with the removal of the magnetic field, the fluid can be returned to its liquid state just as quickly. The degree of change in the viscosity of the MR fluid is proportional to the magnitude of the applied magnetic field.
FIGS. 1 and 2 are a cross sectional view illustrating a conventional shock absorber using an MR fluid and an enlarged cross sectional view depicting a piston assembly shown in FIG. 1, respectively, disclosed in U.S. Pat. No. 5,284,330 entitled "MAGNETORHEOLOGICAL FLUID DEVICE" issued on Jan. 11, 1994.
The shock absorber 10 comprises two principal components: a housing 20 and a piston assembly 30. The housing 20 includes a volume of magnetorheological (MR) fluid. The fluid includes carbonyl iron particles suspended in silicone oil.
The housing 20 is generally of a cylindrical tube with a first closed end 22. A cylindrical sleeve 25 may be affixed to an inner cylinder by any conventional means, e.g., press fit, welding or adhesive to increase the cross-sectional surface area of the housing 20. The cylinder is closed at a second end thereof by an end member 26. A first seal 27 extends about an outer periphery of the end member 26 to prevent fluid leakage between the housing 20 and the end member 26. A second annular seal 28 is housed in a groove in an inner periphery of the end member 26 and seals against a piston rod 32. A scraper 29 can be used to wipe the MR fluid off the surface of piston rod 32 so as to minimize the loss of MR fluid past the second annular seal 28.
The housing 20 is provided with a floating piston 21 to separate the MR fluid from a pressurized accumulator 23. The pressurized accumulator 23 is necessary to accommodate a fluid displaced by the piston rod 32 as well as to allow for thermal expansion of the fluid.
The piston assembly 30 is shown in greater detail in FIG. 2. A piston head 34 is spool shaped with an upper outwardly extending flange 36 and a lower outwardly extending flange 38. A coil 40 is wound upon the spool-shaped piston head 34 between the upper flange 36 and the lower flange 38. The piston head 34 is made of a magnetically permeable material, such as low carbon steel. Guide rails 42 are attached around an outside of the piston head 34 at particular intervals. As shown in FIGS. 1 and 2, four guide rails 42 are shown spaced uniformly about a periphery of the piston head 34.
An electrical connection is made to the coil 40 through the piston rod 32 by lead wires 45 and 47. The first lead wire 45 is connected to a first end of an electrically conductive rod 48 which extends through the piston rod 32 to a Phono-jack connector 46. A center connection of the Phono-jack 46 is connected to a first end 39 of the coil 40. A second end 41 of windings of the coil 40 is attached to a "ground" connection on an outside of the Phono-jack 46. An electrical return path, then, includes the piston rod 32 and the ground lead 47.
However, such an MR shock absorber has some inherent drawbacks. First, since the MR fluid has a higher viscosity than the conventional hydraulic fluid even in the absence of a magnetic field, the MR shock absorber tends to exert a harder damping force against external forces applied thereto, resulting in a deteriorated ride-comfort under certain circumstances. Furthermore, the MR fluid is costly, increasing the manufacturing cost of the shock absorber.